Abstract:

SUMMARY: Late Precambrian tillites in western Spitzbergen, which were metamorphosed and strongly deformed during mid-Palaeozoic and mid-Cenozoic orogenies, contain stones up to 1 m or more in diameter, generally set in a fine-grained psammitic and pelitic matrix. The stones are lithologically varied, and include dolostone, limestone, quartzite, granite, granite-gneiss, volcanic rocks and sandstones. The tillites illustrate the varied rheological behaviour of different rock types under the same TP conditions. Where stones are dispersed through the matrix the degree of deformation appears to follow the pattern: granite (unfoliated) < dolostone < quartzite < metavolcanic rocks < granite-gneiss < limestone, with axial ratios for granite being around 2: 1 and for limestone over 20: 1. Matrix composition has a marked influence on the degree of deformation undergone by the stones. For example, stones in a sandy matrix are more deformed than those in finer-grained matrix of an adjacent horizon, suggesting that the latter is more capable of taking up the regional strain. Similarly, the deformation of stones is greater in rocks bearing only a small amount of matrix.

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